WO2005017604A1

WO2005017604A1 - Coherence reducer

Info

Publication number: WO2005017604A1
Application number: PCT/EP2004/008353
Authority: WO
Inventors: Alexander Pesch; Kristina Uhlendorf; Peter Kuschnerus; Arnaud Deparnay
Original assignee: Carl Zeiss Sms Gmbh
Priority date: 2003-08-04
Filing date: 2004-07-26
Publication date: 2005-02-24
Also published as: DE10335670A1

Abstract

A coherence reducer for a beam with a given temporal coherence length is disclosed, which can be embodied as a Michelson interferometer in which at least one 0° mirror is replaced by a partially transparent mirror and one further mirror, the optical separation of which corresponds to at least half the temporal coherence length.

Description

coherence-reducing

The invention relates to a coherence reducer for a beam with a predetermined temporal coherence length.

Such coherence reducers are used, for example, in the illumination unit of inspection microscopes for semiconductor production if the illumination unit uses, for example, ikrolens arrays for uniform illumination, which can cause disturbing interferences due to the coherence of the beam. Unwanted interference can also occur with other lighting and projection devices that use a beam with a predetermined time coherence length.

Proceeding from this, it is an object of the invention to provide a coherence reducer for a beam with a predetermined temporal coherence length, the coherence reducer being designed to be simple and at the same time easily adaptable to different temporal coherence lengths.

According to the invention, the object is achieved by a coherence reducer for a beam having a predetermined temporal coherence length, the coherence reducer having a first arm and a coupling-in device which couples a first main beam into the first arm from the beam, the arm having a reflection module which has the first Main beam is reflected back to the coupling-in device, and the coupling-out device forms an outgoing beam from the back-reflected main beam, the reflection module of the first arm in the direction of propagation of the first main beam coming from the coupling / decoupling device having a first semitransparent mirror, which forms part of the first main beam as the first partial beam EinJAuskoppelvorrichtung

^" reflected back, and comprises an end mirror whose optical distance from the first partially transparent mirror corresponds to at least half the time coherence length and the at least part of the first transmitted through the first partially transparent mirror Main beam reflected back over the first partially transparent mirror to the coupling-in device as an end mirror partial beam such that the end mirror partial beam is superimposed on the first partial beam.

Since the optical distance between the partially transparent mirror and the end mirror corresponds to at least half the time coherence length, the end mirror part beam and the first part beam are no longer capable of interfering with one another. The disturbing interference effects can thus be reduced.

The time coherence length is understood here to mean the coherence length in the direction of propagation of the beam (or a minimum, preferably the first minimum of the time coherence function).

In particular, the mirrors of the reflection module of the first arm are designed and aligned such that each partial beam is superimposed, in particular coaxially, by the first-order partial beams reflected back from the mirrors to the coupling-in device. (A back-reflected partial beam of the first order is understood to mean any beam reflected at one of the mirrors that was reflected exactly once in the reflection module.) The beam diameter of the superimposed partial beams is practically hardly enlarged, so that the emerging beam is essentially the same

Beam diameter and the same divergence as the incident beam can have. The

Coherence reducer thus generates an outgoing beam with essentially the same beam properties from the incident beam, except for the fact that the interference capability is significantly reduced. Therefore, the coherence reducer according to the invention can, if there is sufficient space, be interposed in an existing optical system without having to change the optical system.

In a preferred embodiment of the coherence detector according to the invention, at least one further partially transparent mirror (or even several) is arranged between the first partially transparent mirror and the end mirror in such a way that the optical distance between two adjacent mirrors corresponds in each case to at least half the time coherence length. The beam reflected back from the reflection module of the first arm to the coupling / decoupling device (the superposition of the partial beams) thus contains several components which are no longer capable of interference with one another. Of course, the distance of the mirrors from mirror to mirror can increase (for example, double). This is possible in the direction away from or towards the input coupling device. Furthermore, in the coherence reducer, the reflection coefficients of the semitransparent mirrors of the reflection module of the first arm can be chosen so that in each case with two adjacent mirrors, the mirror closer to the coupling-in device has a smaller reflection coefficient than the other semitransparent mirror. In particular, the reflection coefficients of the semitransparent mirrors and the reflection coefficient of the end mirror can be so _. be chosen so that the partial beams of the first order reflected from the mirrors to the coupling / decoupling device each have approximately the same intensity. This produces a very homogeneous beam, which is of course very advantageous in the case of lighting devices.

A particularly preferred development of the coherence detector according to the invention consists in that two adjacent mirrors of the reflection module of the first arm are formed on interfaces of a body which is transparent to the beam. On the one hand, this can be produced very easily and with high accuracy. On the other hand, these two mirrors, which are formed on the transparent body, are then easy to adjust, since the adjustment of the two mirrors to one another was already carried out during manufacture.

The reflection coefficient of at least one of the mirrors of the reflection module of the first arm can be designed to be constant over the cross section of the incident beam. This can be easily produced and leads to a reflected partial beam which has a uniform intensity over its entire beam cross section.

At least one of the mirrors can be designed as a Bragg reflector, which alternately comprises λ / 4 layers of high-index and low-index materials. With such Bragg reflectors, desired reflectivities for predetermined wavelengths can now be set with extremely high accuracy.

However, it is also possible for at least one of the mirrors of the reflection module of the first arm to have reflective and transmissive sections, the proportion of the reflective sections being selected as a function of the degree of reflection. The reflective sections can also be partially transparent, so that the reflection coefficient is then determined both via the proportion of the area of the partially reflective sections and their partial reflectivity.

The coherence reducer can furthermore have a second arm, the coupling-in / coupling-out device dividing the beam into first and second main beams such that the first main beam is coupled into the first arm and the second main beam into the second arm, the second arm being coupled in Reflection module that reflects the second main beam back to the EinJAoppeloppelvorrichtung, and the EinJAusoppelvorrichtung forms an outgoing beam from both back-reflected main beams. Thus, according to the invention, a conventional interferometer, for example a Michelson interferometer, can be modified such that at least one 0 ° mirror is replaced by the reflection module with the first semitransparent mirror and the end mirror.

A particularly preferred embodiment of the coherence reducer according to the invention consists in that the reflection module of the second arm is designed in the same way as the reflection module of the first arm. This results in a further reduction in the interference ability of the emerging beam. In particular, the optical distance between the first semitransparent mirror of one of the two reflection modules and the coupling / decoupling device is greater than the optical distance between the end mirror of the other reflection module and the coupling / decoupling device by at least half the time coherence length. This ensures that the partial beams of the two reflection modules are not capable of interference with one another.

The mirrors of the reflection module of the first and / or second arm can be designed as plane mirrors which are aligned parallel to one another. This creates an ideal coaxial superposition of the reflected partial beams. Alternatively, it is also possible that at least one of the plane mirrors is tilted somewhat relative to the other plane mirrors, so that the reflected partial beams are laterally (transversely to the direction of propagation) slightly offset from one another or diverge somewhat. The tilt relative to the parallel orientation is preferably less than 1 °. In addition to the reduction in the interference capability, this also achieves homogenization in the emerging beam.

In particular, the reflection module of the second arm can in turn also be designed as a Michelson interferometer, in which at least one of the two 0 ° mirrors is designed in the same way as the reflection module of the first arm.

The coherence reducer can be designed such that the direction of propagation of the beam with the predetermined time coherence length and of the emerging beam are different and preferably enclose an angle of 90 °.

Furthermore, the coherence reducer described above can be used in an illumination device, in particular a microscope illumination device, the coherence reducer being followed by an illumination optical system which illuminates a plane to be illuminated with the emerging beam. The illumination optics can in particular have two microlens arrays arranged one behind the other and a condenser optics arranged downstream of the microlens arrays. Other lighting optics are also possible. Thus, a lighting device, in particular a

Microscope illumination device provided, in which undesired interference in the field to be illuminated is suppressed (preferably completely suppressed).

A further development of the use of the coherence reducer in a lighting device consists in that at least one diffusing screen is arranged in front of and / or behind the coherence reducer, through which the incident beam or the emerging beam passes. With this diffuser, excellent homogeneity can be achieved in the plane to be illuminated. It has proven to be particularly advantageous if a stationary and a moving lens, which are arranged one behind the other, are provided, this arrangement preferably being arranged in front of the coherence reducer, so that the incident beam passes through these two lenses. Of course, the arrangement can also be used behind the coherence reducer.

Furthermore, a method for reducing the coherence of a beam having a predetermined temporal coherence length is provided, wherein a first main beam is reflected back in itself from the beam, an outgoing beam being formed from the back-reflected main beam, part of the first main beam after passing through a first optical path length as first partial beam is reflected back and another part is transmitted and at least a part of the transmitted first main beam after passing through another optical. Path length, which corresponds to at least half the time coherence length, is reflected back as the end mirror partial beam in such a way that the end mirror partial beam is superimposed (preferably coaxially) with the first partial beam.

A further development of the method is that the beam with the predetermined temporal coherence length is divided into a first and a second main beam, which are reflected back after passing through different optical paths. In particular, the method can be further developed as it can be implemented according to the coherence reducer according to the invention and its further developments.

As a radiation source that emits the beam with the predetermined time coherence length, lasers such as. e.g. Excimer lasers (e.g. krypton fluoride, argon fluoride or fluoride excimer lasers) can be used.

The invention is explained in more detail below by way of example with reference to the drawings. Show it: 1 schematically shows a lighting device which contains a coherence reducer according to the invention, and FIG. 2 shows a detailed view of the coherence reducer from FIG. 1.

The lighting device shown in Fig. 1 comprises the coherence reducer 1, into which a laser beam S1 with a predetermined temporal coherence length is coupled (seen in Fig. 1 from the left, indicated by the arrows P1 and P2) and which emits an outgoing beam S2, the latter Interference ability is reduced. The outgoing beam S2, which is emitted downward in FIG. 1, is again indicated by two arrows P3 and P4, wherein, as can be seen from FIG. 1, the beam cross section of the outgoing beam S2 essentially corresponds to the beam cross section of the incident beam S1 , In the example described here, the laser beam S1 is generated by an excimer laser (not shown) and has a wavelength of 157 nm and a temporal coherence length of 15 mm.

The lighting device further contains two spaced-apart microlens arrays 2, 3, each of which has a plurality of microlenses arranged in rows and columns (only three are shown in the figure). The distance between the two microlens arrays 2, 3 corresponds approximately to the focal length of the microlenses of the array 3. The two microlens arrays 2 and 3 are followed by a condenser lens system 4, which is arranged schematically by the lens shown, and which is separate from the individual microlenses of the microlens array 3 outgoing partial beams onto the field F to be illuminated, in order to illuminate it as uniformly as possible.

If the distance between the two microlens arrays 2, 3 does not exactly correspond to the focal length of the microlenses of the array 3, there is a slight defocusing of the arrangement, which leads to better illumination of the pupil (conjugate plane to field F). The focal points of the microlenses are found in the pupil, the focal points being spread out due to the slight defocusing and the better illumination being achieved as a result. The microlenses of the two arrays 2 and 3 can have the same or different refractive indices. There may also be other differences between the two microlens arrays 2 and 3. It is important that the illumination optics focus a parallel beam in the field plane F and that the plane of the first microlens array 2 is imaged in the field plane F. It is also possible to dispense with the second microlens array 3.

The structure of the coherence reducer 1 can be seen from FIG. 2. The coherence reducer 1 contains an input coupling device 5, which here is a dividing device which is designed as a partially transparent plate which is inclined at 45 ° with respect to the direction of the incident beam S1 and thus converts the incident beam S1 into a first main beam H1, which is shown in FIG the first arm A1 is coupled, and splits the second main beam H2, which is coupled into the second arm A2.

In the first arm A1, two bodies K1 and K2 (here made of calcium fluoride) which are transparent to the laser radiation are arranged at a distance from one another, the end faces 6, 7, 8 and 9 of which are mirrored, so that four mutually spaced plane mirrors 6 to 9 are arranged in the first arm A1 are. The optical distance between two adjacent plane mirrors 6, 7; 7, 8 and 8, 9 is chosen so that it is at least as long as half the time coherence length of the beam S1 (here at least 7.5 mm). The mirrors 6 to 8 are each designed as partially transparent mirrors, so that the main beam H1 is first partially reflected on the first mirror 6, so that a first partial beam T11 is generated. The transmitted part of the first main beam H1 is then partly reflected again on the second partially transparent mirror 7, so that a second partial beam, T12, is generated. The same then takes place on the third partially transparent mirror 8, on which the third partial beam T13 is generated, and the part of the main beam H1 transmitted up to the end mirror 9 is reflected on the end mirror 9 (as completely as possible), as a result of which the end mirror partial beam T14 is generated. Since the plane mirrors 6 to 9 are aligned parallel to each other, the rays are always reflected back in themselves. The partial beams T11 to T14, which are reflected back, are shown somewhat offset from the main beam H1 only for the sake of clarity in the drawing. Multiple reflections are not taken into account here. that the reflected partial beams are first order partial beams (that is, they were reflected exactly once). The partial beams T11 to T14 are coaxially superimposed by the reflection on the plane mirrors 6 to 9, so that the beam reflected back to the divider device 5 (which comprises the partial beams T11 to T 14) has a significantly reduced interference capability. The body K1 and K2 form with the mirrors 6 to 9 a reflection module R1, which reduces the interference ability of the first main beam H1.

The reflection coefficients (or reflectance) of the mirrors 6 to 9 are set so that each partial beam T11 to T14 has approximately the same intensity. If multiple reflections and absorptions are not taken into account, the reflection coefficients of the remaining semitransparent mirrors 8 to 6 can be calculated on the basis of the reflection coefficient of the end mirror 9 according to the following formula:

If you number the mirrors from the end mirror 9 to the divider device 5 (i.e. the end mirror 9 is the first mirror, the divider mirror 8 is the second mirror) Divider mirror 7 is the third mirror and divider mirror 6 is the fourth mirror), starting from the reflection coefficient of the nth mirror, the reflection coefficient of the n + 1th mirror can be calculated according to the above formula (R _n is the reflection coefficient of the nth mirror and R _{n + 1} is the reflection coefficient of the n + 1th mirror). In the example described here, the reflection coefficient of the end mirror 9 was specified as 0.93. It follows from this that the reflection coefficient of the second mirror (partially transparent mirror 8) is 0.37, the reflection coefficient of the third mirror (partially transparent mirror 7) is 0.22 and the reflection coefficient of the fourth mirror (partially transparent mirror 6) is 0.15 (in each case two digits calculated according to the above formula). If one also takes into account absorption effects for the example described here (the material of the bodies K1 and K2 is calcium fluoride), a linear system of equations is obtained, which can be solved at least numerically. As a result, 0.364, 0.221 and 0.155 are then obtained for the reflection coefficients of mirrors 8 to 6. This shows that a very good estimate can be made with the given formula.

The mirrors 6 to 9 are each here as a Bragg mirror (layer system of λ / 4 layers stacked on top of one another with alternating high and low refractive indices). LaF ₃ can be used as the material with high refractive index and MgF ₂ as material with low refractive index.

The second arm A2 of the coherence reducer is essentially identical to the first arm A1, the transparent bodies K3 and K4 corresponding to the transparent bodies K1 and K2. The mirroring of the end faces of the transparent bodies K3 and K4 to form the mirrors 10, 11, 12 and 13 can also be identical to the mirrors 6 to 9. (The bodies K3 and K4 with the mirrors 10 to 13 form a second reflection module R2). However, the optical distance between the partially transparent mirror 10 and the divider device 5 is selected such that it is at least half the time coherence length of the beam S1 greater than the optical distance between the mirror 9 and the divider device 5 (in FIG. 2, the distance is to simplify the illustration) not shown exactly). The optical distance of the partially transmissive mirror 10 from the divider device is therefore selected such that it is greater by at least half the time coherence length of the beam S1 plus the total optical length of the first reflection module R1. This ensures that the partial beams T21, T22, T23 and T24 are not capable of interference with the partial beams T11 to T14.

Of course, the two arms do not have to be of the same design with regard to the mirrors. So in both arms A1 and A2 a different one. Number of mirrors can be provided. Furthermore, it is also possible to provide a second divider device in the second arm instead of the two bodies K3 and K4, which in turn divides the main jet H2 into two Splits beams and couples them into two different arms, each of which can be constructed as well as arm A1.

Furthermore, it is also possible that the plane mirrors 6 to 9 and 10 to 13 are not aligned exactly parallel to one another. They can be partially tilted towards each other, so that there is a slight lateral offset of the superimposed partial beams T11 to T14 and T21 to T24, as a result of which homogenization is achieved across the beam cross section.

The position of the bodies K1 and K2 and / or K3 and K4 can be changed in the beam spreading device in order to adapt the coherence reducer 1 to other temporal coherence lengths. Of course, the mirrors 6 to 9 and 10 to 13 do not have to be formed on the end faces of the bodies K1 and K4, but can also be designed as individual mirrors.

The partial beams T11 to T14 and T21 to T24 reflected back by the two arms are (at least partially) superimposed on the divider device 5 and form the emerging beam S2, the interference capability of which is significantly lower due to the coherence reducer.

The coherence reducer can be used for partially coherent and coherent radiation sources in the wavelength range from infrared to extreme UV (wavelength of about 13 nm), in particular for inspection microscopes (their illumination unit), e.g. B. in semiconductor manufacturing, lithography devices, laser projection systems and also with polarized lighting for measurement and projection systems.

Claims

claims

1. Coherence reducer for a beam (S1) with a predetermined temporal coherence length, wherein the coherence reducer (1) has a first arm (A1) and a coupling / decoupling device (5) which emits a first main beam (H1) from the beam (S1). couples into the first arm (A1), the arm (A1) having a reflection module (R1, R2) which reflects the main beam (H1) back to the coupling-in device (5), and the coupling / decoupling device (5) from the back-reflected main beam forms an outgoing beam (S2), the reflection module (R1) of the first arm (A1) in the direction of propagation of the first main beam (H1) coming from the coupling-in device (5) forming a first semitransparent mirror (6) which forms part of the first main beam ( H1) is reflected back as the first partial beam (T11) to the input coupling device (5), and comprises a first end mirror (9), the optical distance of which from the first partially transparent mirror (6) is at least half the time hen corresponds to coherence length and the at least part of the first main beam (H1) transmitted by the first partially transparent mirror (6) reflects back via the first partially transparent mirror (6) to the coupling-in device (5) as an end mirror partial beam (T14) such that the end mirror partial beam (T14) co-exists the first partial beam (T 1) is superimposed.

2. Coherence reducer according to claim 1, wherein each partial beam of the partial beams (T11, T12, T13, T14) reflected back from the mirrors (6-9) to the input coupling device (5) is respectively superimposed with the remaining partial beams, in particular coaxially.

3. Coherence reducer according to one of the above claims, wherein at least one further partially transparent mirror (7, 8) is arranged between the first partially "transparent" mirror (6) and the end mirror (9) such that in each case the optical distance between two adjacent mirrors (6, 7; 7, 8; 8, 9) corresponds to at least half the time coherence length.

4. Coherence reducer according to claim 3, wherein the reflection coefficients in the partially transparent mirrors are selected such that in each case with two adjacent mirrors (6, 7; 7, 8) the mirror (6, 7) closer to the coupling-in device (5) has a smaller one Has reflection coefficient than the other semitransparent mirror (7, 8).

5. coherence reducer according to claim 4, wherein in the end mirror (9) and the semitransparent mirrors (6-8) the reflection coefficients are selected such that the partial beams (T11-T14) reflected back by the mirrors (6-9) to the coupling-in device (5) first order each have the same intensity.

6. Coherence reducer according to one of the above claims, wherein two adjacent mirrors (6, 7; 7, 8; 8, 9) of the reflection module (R1) of the first arm (A1) on interfaces of a body transparent to the first main beam (H1) ( K1, K2) are formed.

7. Coherence reducer according to one of the above claims, wherein the reflection coefficient of at least one of the mirrors (6-9) is constant over the cross section of the incident beam.

8. Coherence reducer according to one of the above claims, in which at least one of the mirrors (6-9) is designed as a Bragg reflector.

9. Coherence reducer according to one of the above claims, in which at least one of the mirrors (6-9) has reflective and transmissive sections, the proportion of the reflective sections being selected as a function of the degree of reflection.

10. Coherence reducer according to one of the above claims, in which the mirrors (6-9) are designed as plane mirrors and are arranged parallel to one another.

11. Coherence reducer according to one of claims 1 to 10, in which the mirrors (6-9) are designed as plane mirrors and two of the mirrors are tilted towards one another, preferably by less than 1 °.

12. Coherence reducer according to one of the above claims, wherein the coherence reducer (1) has a second arm (A2) and the EinJAoppeloppelvorrichtung (5) divides the beam (S1) into a first and second main beam (H1, H2) so that the first The main beam (H1) is coupled into the first arm (A1) and the second main beam (H1) into the second arm (A2), the second arm (A2) having a reflection module (R2) which is the second main beam (H2) A coupling-out device (5) reflects back, and the coupling-out device (5) forms an outgoing beam (S2) from the two main beams reflected back.

13. Coherence reducer according to claim 12, in which the reflection module (R2) of the second arm (A2) is designed in the same way as the reflection module (R1) of the first arm (A1).

14. Coherence reducer according to claim 13, wherein the optical distance between the first partially transparent mirror (6, 10) of one of the reflection modules (R1, R2) and the coupling / decoupling device (5) is greater than the optical length by at least half the time coherence Distance between the end mirror (13, 19) of the other reflection module (R2, R1) and the EinJAuschoppelvorrichtung (5).

15. Use of the coherence reducer according to one of the above claims in an illumination device, in particular a microscope illumination device, which comprises a coherence reducer (1) downstream illumination optics (2, 3, 4) that direct the outgoing beam (S2) onto a plane to be illuminated (F ) maps.

16. Use according to claim 15, in which a diffuser is arranged in front of and / or behind the coherence reducer, through which the incident beam (S1) or the emerging beam (S2) passes,

17. A method for reducing the coherence of a beam with a predetermined temporal coherence length, the beam reflecting back a first main beam in such a way that part of the first main beam is reflected back as a first partial beam after passing through a first optical path length and another part is transmitted and at least one Part of the transmitted first main beam after passing through a further optical path length, which corresponds to at least half the time coherence length, is reflected back as an end mirror partial beam, the end mirror partial beam being superimposed on the first partial beam and an emerging beam being formed therefrom.